(a) Field of the Invention
The present invention relates to a reflective type liquid crystal display, in particular, to a bistable twisted-nematic mode reflective liquid crystal display.
(b) Description of the Related Art
Reflective type liquid crystal displays (LCDs) have been attracting users recently, especially in the area of low power consumption applications such as portable information communication devices. A conventional reflective type LCD includes a liquid crystal cell that has two facing substrates with field generating electrodes and a liquid crystal layer therebetween. The reflective type LCD usually includes two polarizers attached to the outer surfaces of the liquid crystal cell and a reflector behind the liquid crystal cell.
However, typical reflective type LCDs suffer from low brightness and parallax. Such problems become more severe in color displays with color filters for individual pixels.
One of the solutions is to use a mirror electrode just behind the liquid crystal layer and a single polarizer. [See S.-T. Wu and C.-S. Wu, Applied Physics Letter 68, 1455 (1996), C.-L. Kuo and C.-K. Wei, Proceedings of EuroDisplay 96, 387 (1996). and T. Uchida et al., Asia Display 95, p. 599 (1995).]
However, the conventional reflective type LCD with a single polarizer has a problem of low contrast.
It is therefore an object of an embodiment of the present invention to increase the contrast ratio of a reflective type LCD with a single polarizer.
These and other objects are provided, according to an embodiment of the present invention, by using a bistable twisted nematic mode.
In detail, a liquid crystal display according to an embodiment of the present invention includes front and rear insulating substrates, each having an inner surface and an outer surface. The inner surfaces of the two substrates face each other. A transparent electrode is formed on the inner surface of the front substrate, and a mirror electrode made with a reflective material is formed on the inner surface of the rear substrate. A polarizer is attached on the outer surface of the front substrate, and homogeneously aligning films are prepared on both the transparent and the reflective electrodes. A bistable liquid crystal layer is interposed between the two substrates with the aligning films. The liquid crystal layer includes a nematic liquid crystal material having positive dielectric anisotropy and a chiral dopant so as to produce a twisted structure. The molecular director in the liquid crystal layer is driven in such a way that it has two stable states.
The two stable states may correspond to the twist angle of "PHgr" xc2x1xcfx80, respectively, where "PHgr" is an initial twist angle of the molecular director. The first stable state, "PHgr"xe2x88x92xcfx80, is less twisted and the second stable state, "PHgr"+xcfx80, is more twisted than the initial twisted state.
The thickness (d) of the liquid crystal layer, the natural pitch (p) of the liquid crystal, and the initial twist angle ("PHgr") of the molecular director preferably satisfy the relationship of 1.2("PHgr"/2xcfx80)xe2x89xa6d/pxe2x80x94xe2x89xa61.4("PHgr"/2xcfx80). In this case, it is preferable that the initial twist angle of the molecular director is substantially equal to xcfx80.
The homogeneously aligning films are preferably rubbed in two different directions The rubbing direction on one of the films is antiparallel to that on the other. It is desirable that the optic axis of the polarizer makes 45 degrees with respect to the two rubbing directions. If the optical retardation through the liquid crystal layer in the first stable state is equal to a quarter wavelength of the incident light, the light reflected from the mirror is blocked by the polarizer while it is transmitted through the polarizer in the second stable state which is optically isotropic.
According to an embodiment of the present invention, the LCD further includes a retardation film interposed between the front substrate and the polarizer. In one embodiment of the present invention, the optic axis of the retardation film of a quarter wavelength of the incident light is preferably perpendicular to the two rubbing directions. Such retardation film converts a linearly polarized light into a circularly polarized one. In this case, if the optical retardation through the liquid crystal layer in the first stable state is equal to a quarter wavelength of the incident light, the light reflected from the mirror becomes transmitted through the polarizer since the optical retardation is exactly compensated by the retardation film. On the other hand, in the second stable state, the reflected light is blocked since its polarization is perpendicular to the optic axis of the polarizer through total retardation of a half wavelength. In another embodiment of the present invention, the optic axis of the retardation film is parallel to the two rubbing directions.
According to an embodiment of the present invention, the first and the second stable states which undergo bistable switching are obtained by sequentially applying a reset voltage and a selection voltage, which is lower than the reset voltage, to the LCD cell. The rest voltage makes the molecular director to be oriented homeotropically from an initial, homogeneously twisted structure, and the selection voltage makes the molecular director to be stable in one of the first and the second states. A data voltage, lower than the selection voltage, that follows subsequently the reset and the selection voltages may be applied across the transparent electrode and the mirror electrode.